I have solved the problem by using the UB Ports installer on a VM (Mint hosted on Devuan). Both my Devuan PC and the VM recognized the device during normal and fastboot modes with no trouble, and all I had to do was redirect the USB connection to the VM a few times as the device restarted during installation. My Xperia is now running Ubuntu Touch with zero problems.

If I ever do this in the future, I'll avoid the Windows version of the installer completely. I've used it twice now and both times it has been more hassle than using a Linux version on a compatibility-focused distro like Mint.